Mechanical deaeration is the first line of defense against oxygen. If it underperforms, the result can be very costly.
In Part I of this series, we saw that dissolved O₂ and CO₂ are the primary corrosive agents in steam generation systems and that their removal is based on Henry's Law. In this part, we cover the types of deaerators available, selection criteria, and the operational control points that make a difference in practice.
Types of Deaerators
There are three configurations in most common use in steam generation systems: Spray, Tray, and Combined Spray-Tray.
Spray Type
Water is atomized into fine droplets by spray nozzles, increasing contact area with the steam and making it easier to remove dissolved gases.
- Advantages: simple design, lower initial cost, accessible maintenance and compact footprint.
- Limitations: lower efficiency at very low O₂ concentrations; sensitive to nozzle wear and clogging.
- Typical application: smaller boilers and lower-pressure operating systems.
Tray Type
Water flows over perforated trays in a thin film while steam rises countercurrent to it. The large surface area and intense steam-liquid contact provide significantly better mass transfer than spray alone.
- Advantages: higher O₂ removal efficiency, superior operational stability and lower sensitivity to load variations.
- Limitations: more complex design, higher initial cost and larger footprint required.
- Typical application: medium- and high-pressure boilers where an O₂ specification below 7 ppb must be met consistently.
Combined Spray-Tray Type
The predominant configuration in industrial plants with more stringent feedwater quality requirements. It operates in two sequential stages:
- Spray stage: preheats the water and removes most of the dissolved gases (O₂ and CO₂), leveraging the simplicity of spraying for the coarser work.
- Tray stage: performs the final polishing, removing residual O₂ to reach and maintain levels consistently below 7 ppb.
- Advantages: very high efficiency, better performance under variable loads.
- Disadvantages: greater construction complexity and higher cost.
- Typical application: thermal power plants, high-pressure boilers and critical steam generation systems. It is the type most frequently found in medium and large plants.
Regardless of type, a deaerator can operate at atmospheric pressure or slightly above (typically 0.07 to 0.3 bar g, corresponding to 105–117°C). Pressurized deaerators are more efficient because the higher saturation temperature shifts the solubility equilibrium more effectively, resulting in lower and more stable residual O₂ levels.
Selection Criteria
Choosing the right type and sizing of deaerator depends on jointly evaluating:
- Boiler operating pressure
- Feedwater flow rate and load variation
- Inlet water temperature and O₂ content
- Outlet O₂ specification
- Steam availability and quality
- Capacity and residence time of the deaerator storage tank
Diagnosing Common Failures
- High O₂ with OK temperature: Likely cause — restricted vent or clogged spray nozzles. Check: visual inspection of the vent and steam flow through the purge.
- High O₂ with vent open: Likely cause — operating temperature below saturation. Check: verify operating pressure and inlet delta T.
- Erratic O₂ variation: Likely cause — load variation without stable pressure control. Check: install pressure control on the deaerator.
- High O₂ even with scavenger dosed: Likely cause — error in the measurement system or sample collection. Check: review the sampling system per ASME CRTD-81.
- High iron in condensate: Likely cause — corrosion in the deaerator storage section. Check: inspect internal surface passivation and the treatment program.
Economic Impact
Deaerator malfunction typically generates costs on four fronts: higher fuel consumption, higher chemical treatment consumption, higher corrosion-related maintenance, and unplanned shutdowns.
A documented public case shows that simply adjusting deaerator operation at an oil sands plant resulted in savings of USD 23.4 million per year, with a significant reduction in water and fuel consumption. [watertechsolutions.com]
Final Considerations
A well-selected deaerator, operating within the correct parameters, with proper venting, stable temperature and a reliable measurement system, is capable of keeping dissolved O₂ consistently below 7 ppb.
Most of the time, the problem isn't the equipment — it's the operation.
In Part III, the topic will be oxygen scavengers.
References
- ASME PTC 12.3. Performance Test Code on Deaerators.
- HEI. Standards for Deaerators.
- NALCO. Water Handbook. 3rd ed.
- Veolia/Syncrude. Case Study — Deaerator Optimization. Available at: watertechsolutions.com.